A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. Clathrates, or gas hydrates, are crystalline water-based solids in which small non-polar molecules (typically gases) are trapped inside “cages” of hydrogen bonded, frozen water molecules. Gas hydrates are found in Arctic and Antarctic ice sheets, where air trapped within snow becomes a stable (air) gas hydrate at high-depth and low-temperature conditions. Nitrogen gas and water can form a nitrogen gas hydrate in which a large amount of nitrogen is trapped within a crystallized water lattice.
Gas hydrates are typically stable under low-temperature and high-pressure conditions. Gas hydrate formation is problematic for the oil and gas industry in deep water drilling operations, owing to low temperature and high pressure conditions that favor the formation of gas hydrates.
For example, the National Oceanic and Atmospheric Administration (NOAA) provides ocean temperatures at depth for various locations in the Pacific Ocean as well as other locations (McPhaden 1999). Typically, ocean depth can be divided into three vertical layers. The top layer is the surface layer, or mixed layer. Water temperature is highest in the surface layer, and is easily influenced by solar energy, wind, and rain. The next layer is the thermocline, where water temperature drops rapidly as the depth increases. The lowest layer is the deep-water layer. Water temperature in this zone decreases slowly as depth increases. Water temperature in the deepest parts of the ocean averages about 36° F. (2° C.).
Hydrostatic pressure at depth h beyond the water's surface is given by the formula p=po+ρ·g·h, where po is atmospheric pressure at sea level, ρ is the density of water, and g is gravitational acceleration. In order to operate at drilling depths, BOP control systems are designed to operate against extreme hydrostatic pressures. BOP accumulators are charged with up to 10,000 pounds per square inch (psi) of gas, typically nitrogen gas, to provide hydraulic actuating force for BOP control systems. In some examples, BOP control system pressure is 3,000 psi+the hydrostatic pressure of the seawater above it.
Gas hydrates may unexpectedly form within BOP control fluid at the temperature and pressures encountered by BOPs in deep water drilling operations. Gas hydrate formation is exacerbated when drilling in colder arctic waters, which occurs at temperatures near the freezing point of water. Nitrogen gas hydrate formation in subsea BOP control systems is believed to be the result of nitrogen gas entrapped in the tubing, or escaped from the subsea accumulator bottles. The formation of nitrogen gas hydrates, or nitrogen hydrates, in the control system of a BOP is consistent with the loss of control of the BOP. There is a need in the industry for methods and compositions to prevent the formation of gas hydrates within BOP control systems.